The concept of light-induced retinal damage has been recognized since the time of Plato, and was first clinically studied as early as 1916 as discussed in an article titled "Pathological Effects of Radiant Energy on the Eye" by Dr. F. H. Verhoeff, et al. printed in Proc. Am. Acad. Arts Sci, Volume 51, pages 629-818 (1916) . The article describes the nature of solar burns affecting the retinal pigment epithelium and choroid. However, at that time, they attributed this damage to an increase in ocular temperature and not to an overstimulation of the retina with light.
It was not until experimental work performed in the mid 1960's that nonthermal retinal light damage became recognized. The myth that such damage could be caused only by intense light sources, such as the sun and various photocoagulators, gave way to reality that damage from lower intensity light was probable and that additivity of several exposures was possible.
And, in the last twenty years, the prior art is replete in describing how commonly-used ophthalmic devices and microscopes can cause experimental light damage in animal models. In the last fifteen years, the use of intense light sources in ophthalmic surgery has increased. And, recently, there have been numerous reports of iatrogenic phototoxicity following routine cataract extraction, epikeratophakia, combined anterior segment procedures, and vitrectomy surgery.
The term "phototoxicity" or "phototoxic lesion" is usually employed to describe the typical retinal lesion produced after a relatively short intense exposure to a light source such as the operating microscope.
In an article by Dr. D. M. Robertson et al., entitled "Photic Retinopathy from the Operating Room Microscope", printed in the American Journal of Ophthalmology, Volume 101, pages 452.varies.462 (1986), it was conclusively established that a cause and effect relationship exists between exposure to the operating microscope light and retinal or phototoxic lesions. Evidence also exists that light from the operating microscopes contributes to post-operative cystoid macular edema (CME).
A number of factors attribute to the occurrence of phototoxicity or CME in patients during an ophthalmic operation. And, since retinal damage is largely a function of these factors such as light power, exposure time, and wave length of the light, a reduction of total energy delivered to the retina throughout an entire procedure can be accomplished in various manners.
Filtration is one method of limiting the damaging light rays from reaching the retina. Retinal susceptibility to phototoxic effects is greatest at the blue-violet end of the light spectrum. Consequently, filtration that is cut off below 400-450 mm may be added to the microscope illumination system. However, this may be unnecessary where glass fiber bundles or cables are used to transfer light to the microscope because the ultraviolet absorption characteristics of glass cause the glass bundles to provide for a very effective ultraviolet absorption automatically, perhaps making further filtration unnecessary on fiberoptic-equipped microscopes.
Oblique illumination, rather than coaxial illumination, may decrease the power of the light reaching the retina. Oblique illumination will place the intense image of the illuminating beam in the far periphery of the retina, thus protecting the posterior pole. However, the commonly available oblique illumination attachments are somewhat inconvenient, cause vignetting of photo/video images and also increase the length of the microscope body.
Defocusing the illumination beam on the retina by insertion of an air bubble in the anterior chamber will increase the size and, thus, decrease the intensity of the illumination beam as it falls on the retina. However, the amount of defocusing varies greatly with the size of the bubble and it is unclear on whether this procedure would help prevent phototoxicity.
Another manner of preventing phototoxicity is to limit the exposure time the microscope illumination system is projecting its beam of light on a patient's retina. Two methods of minimizing total light energy by decreasing exposure time to the brightest light have been disclosed in the prior art.
First, Dr. Yanoff et al., in an article entitled, "Inexpensive Device to Reduce Surgical Light Exposure", as printed in Ophthalmology, Volume 90, (Instrument and Book Supplement), pages 137-138 (1983), discloses a rubber cap which is placed over the cornea after a cataract removal to cover the visual axis and remains in place during the final closure of the wound. A second approach is discussed in an article by Dr. D.J. McIntyre entitled, "Phototoxicity -- The Eclipse Filter", as printed in Ophthalmology, Volume 92, pages 364-365 (1985), wherein he discusses the use of a movable opaque disk which can be placed in the appropriate plane of the illumination system of the microscope to project a sharply-focused black spot of 11 mm in diameter. The surgeon can place the black spot over the pupil when he does not need direct illumination of the retina and raises the spot away from the pupil when he does need to direct the illumination beam onto the retina. However, both methods use devices which block not only the microscope illumination beam but also the surgeon's sight of the retina during an ophthalmological operation.
It is, therefore, one object of the present invention to provide a microscope having an illumination system or module which decreases the amount of light passing through it over time without interrupting a surgeon's full field of vision through the microscope.
It is another object of the present invention to provide a microscope illumination system having a variable shutter means so as to allow a surgeon to adjust the amount of light that is blocked from striking the patient's retina.
It is a further object of the present invention to provide a microscope having a variable shuttered illumination system capable of reducing the occurrence of phototoxicity in patients undergoing an ophthalmic surgical operation.
It is still a further object of the present invention to provide a microscope having a variable shuttered illumination system capable of reducing the occurrence of postoperative cystoid macular edema (CME) in patients undergoing an ophthalmic surgical operation.
Another object of the present invention is to provide a microscope having a variable shuttered illumination system to reduce the amount of light striking a patient's retina during an ophthalmic surgical operation without restricting the surgeon's field of vision so as to reduce the occurrence of postoperative retinal damage to the patient.